1. Field of the Invention
This invention relates to the production of aluminum alloy sheet for the automotive industry, particularly for body panel applications, having excellent bendability, together with good paint bake response and recyclability.
2. Description of the Prior Art
Various types of aluminum alloys have been developed and used in the production of automobiles, particularly as automobile body panels. The use of aluminum alloys for this purpose has the advantage of substantially reducing the weight of the automobiles. However, introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. In particular, the panels must be able to withstand severe bending, as occurs during hemming operations, without cracking. Hemming is the common way of attaching outer closure sheets to underlying support panels and results in the edges of the sheet being bent nearly back on itself. In addition to this excellent bendability, the aluminum alloy panels, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
Aluminum alloys of the AA (Aluminum Association), 6000 series are widely used for automotive panel applications. It is well known that a lower T4 yield strength (YS), and reduced amount of Fe, will promote improved formability, particularly hemming performance. A lower yield strength can be achieved by reducing the solute content (Mg, Si, Cu) of the alloy, but this has traditionally resulted in a poor paint bake response, less than 200 MPa T8 (0% strain). This poor paint bake response can be countered by increasing the gauge, or by artificially aging the formed panels. However, both of these approaches increase the cost and are unattractive options. Furthermore, a reduced Fe content is not sustainable with the use of significant amounts of scrap in the form of recycled metal. This is because the scrap stream from stamping plants tends to be contaminated with some steel scrap that causes a rise in the Fe level.
Furthermore, the necessary material characteristics of outer and inner panels are sufficiently different that the natural trend is to specialize the alloys and process routes. For example, an AA5000 alloy may be used for inner panels and an AA6000 alloy for outer panels. However, to promote efficient recycling it is highly desirable to have the alloys used to construct both the inner and outer panel of a hood, deck lid, etc. to have a common or highly compatible chemistry. At the very least, the scrap stream must be capable of making one of the alloys, in this case the alloy for the inner panel.
In Uchida et al. U.S. Pat. No. 5,266,130 a process is described for manufacturing aluminum alloy panels for the automotive industry. Their alloy includes as essential components quite broad ranges of Si and Mg and may also include Mn, Fe, Cu, Ti, etc. The examples of the patent show a pre-aging treatment that incorporates a cooling rate of 4xc2x0 C./min from 150xc2x0 C. to 50xc2x0 C.
In Jin et al. U.S. Pat. No. 5,616,189 a further process is described for producing aluminum sheet for the automotive industry. Again, alloys used contain Cu, Mg, Mn and Fe. The aluminum sheet produced from these alloys was subjected to a 5 hour pre-age treatment at 85xc2x0 C. The disclosure furthermore states that the sheet can be coiled at 85xc2x0 C. and allowed to cool slowly to ambient at a rate of less than 10xc2x0 C./hr. The aluminum sheet used in this patent was a continuous cast (CC) sheet and sheet products produced by this route have been found to exhibit poor bendability.
It is an object of the present invention to provide an improved processing technique whereby an aluminum alloy sheet is formed which has excellent bendability.
It is a further object of the invention to provide an aluminum alloy sheet product having good paint bake response.
It is a still further object of the invention to provide an aluminum alloy sheet product which is capable of being recycled for use in the production of automotive body panels.
In accordance with one embodiment of this invention, an aluminum alloy sheet of improved bendability is obtained by utilizing an alloy of the AA6000 series, with carefully selected Mg and Si contents and, with an increased manganese content and a specific pre-age treatment. The alloy used in accordance with this invention is one containing in percentages by weight 0.50-0.75% Mg, 0.7-0.85% Si, 0.1-0.3% Fe and 0.15-0.35% Mn. According to an alternative embodiment, the alloy may also contain 0.2-0.4% Cu.
The procedure used for the production of the sheet product is the T4 process with pre-aging, i.e. T4P. The pre-aging treatment is the last step in the procedure.
The target physical properties for the sheet products of this invention are as follows:
In the above, T4P indicates a process where the alloy has been solution heat treated, pre-aged and naturally aged for at least 48 hours. UTS indicates tensile strength, YS indicates yield strength and E1 indicates total elongation. BEND represents the bend radius to sheet thickness ratio and is determined according to the ASTM 290C standard wrap bend test method. T8 (0% or 2% strain) represents the YS after a simulated paint bake of either 0% or 2% strain and 30 min at 177xc2x0 C.
For Cu-free alloys the functional relationships are revealed which allow the T4P strengths to be related to alloy composition, and the paint bake strength to the T4P strength
The T4P yield strength is given by:
T4P YS (MPa)=130(Mgwt %)+80(Siwt %)xe2x88x9232 
where the T4P is obtained by a simulated pre-age of 85xc2x0 C. for 8 hrs.
The T8 (0% strain) yield strength is given by:
T8 (MPa)=0.9(T4P)+134 
Using these relationships the following alloys will meet the T4P/TB (0%) requirements:
T4P 90 MPa, T8 215 MPa+(0.5 wt % Mg-0.7 wt % Si)
T4P 110 MPa, T8 233 MPa+(0.6 wt % Mg-0.8 wt % Si)
T4P 120 MPa, T8 242 MPa+(0.75 wt % Mg-0.7 wt % Si)
and this gives the nominal composition range for the alloys of the invention of Al-0.5 to 0.75 wt % Mg-0.7 to 0.8 wt % Si.
For Cu containing alloys, the functional relationships are not so straightforward and depend on the Mg and Si content. A Cu content of about 0.2-0.4 wt % is desirable for enhanced paint bake performance.
For reasons of grain size control, it is preferable to have at least 0.2 wt % Mn. Mn also provides some strengthening to the alloy. Fe should be kept to the lowest practical limit, not less than 0.1 wt %, or more than 0.3 wt % to avoid forming difficulties.
For the outer panel the Fe level in the alloy will tend toward the minimum for improved hemming. On the other hand, the Fe level in the alloy for inner panel applications will tend towards the maximum level as the amount of recycled material increases.
The alloy used in accordance with this invention is cast by semi-continuous casting, e.g. direct chill (DC) casting. The ingots are homogenized and hot rolled to reroll gauge, then cold rolled and solution heat treated. The heat treated strip is then cooled by quenching to a temperature of about 60-120xc2x0 C. and coiled. This quench is preferably to a temperature of about 70-100C., with a range of 80-90xc2x0 C. being particularly preferred. The coil is then allowed to slowly cool to room temperature at a rate of less than about 10xc2x0 C./hr, preferably less than 5xc2x0 C./hr. It is particularly preferred to have a very slow cooling rate of less than 3xc2x0 C./hr,
The homogenizing is typically at a temperature of more than 550xc2x0 C. for more than 5 hours and the reroll exit gauge is typically about 2.54-6.3 mm at an exit temperature of about 300-380xc2x0 C. The cold roll is normally to about 1.0 mm gauge and the solution heat treatment is typically at a temperature of about 530-570xc2x0 C.
Alternatively, the sheet may be interannealed in which case the reroll sheet is cold rolled to an intermediate gauge of about 2.0-3.0 mm. The intermediate sheet is batch annealed at a temperature of about 345-410xc2x0 C., then further cold rolled to about 1.0 mm and solution heat treated.
The pre-aging according to this invention is typically the final step of the T4 process, following the solution heat treatment. However, it is also possible to conduct the pre-aging after the aluminum alloy strip has been reheated to a desired temperature.
It has also been found that it is particularly beneficial to conduct the quench from the solutionizing temperature in two stages. The alloy strip is first air quenched to about 400-450xc2x0 C., followed by a water quench.
The sheet product of the invention has a YS of less than 125 MPa in the T4P temper and greater than 250 MPa in the T8(2%) temper. With an interanneal, the sheet product obtained has a YS of less than 120 MPa in the T4P temper and greater than 245 MPa in the T8(2%) temper.
A higher quality sheet product is obtained according to this invention if the initial aluminum alloy ingots are large commercial scale castings rather than the much small laboratory castings. For best result, the initial castings have a cast thickness of at least 450 mm and a width of at least 1250 mm.
With the procedure of this invention, a sheet is obtained having very low bendability (r/t) values, e.g. in the order of 0.2-0, with an excellent paint bake response. Such low values are very unusual for AA6000 alloys and, for instance, a conventionally processed AA6111 alloy sheet will have a typical r/t in the order of 0.4-0.45.
A particularly preferred procedure for producing an aluminum alloy for inner panels applications according to the invention includes DC casting and scalping ingots, then homogenization preheat at 520xc2x0 C. for 6 hours (furnace temp.) followed by 560xc2x0 C. for 4 hours (metal temp.). This is hot rolled to a reroll exit gauge of 2.54 mm with an exit temperature of 300-330xc2x0 C., followed by cold rolling to 0.85 to 1.0 mm. The sheet is then solution heat treated with a PMT of 530-570xc2x0 C. and an air quench to 450-410xc2x0 C. (quench rate 20-75 C./s), followed by a water quench from 450-410 to 280-250xc2x0 C. (quench rate 75-400C./s). Next it is air quenched to 80-90xc2x0 C. and coiled (actual coiling temp.). Thereafter the coil is cooled to 25xc2x0 C. This procedure is described as the T4P practice.
A particularly preferred procedure for producing an aluminum alloy for outer panel applications includes DC casting ingots and surface scalping, followed by homogenization preheat at 520xc2x0 C. for 6 hours (furnace temp.), then 560xc2x0 C. for 4 hours (metal temp.). The ingot is then hot rolled to a reroll exit gauge of 3.5 mm with an exit temperature of 300-330xc2x0 C., followed by cold rolling to 2.1 to 2.2 mm. The sheet is batch annealed for 2 hours at 380xc2x0 C. +/xe2x88x9215xc2x0 C.followed a further cold roll to 0.85 to 1.0 mm. This is followed by a solution heat treat with a PMT of 530-570xc2x0 C., then an air quench to 450-410xc2x0 C. (quench rate 20-75 C./s) and a water quench from 450-410 to 280-250xc2x0 C. (quench rate 75-400xc2x0 C./s). Finally, the sheet is air quenched to 80-90xc2x0 C. and coiled (actual coiling temp.). The coil is then cooled to 25xc2x0 C. This procedure is the T4P practice with interanneal.